Over the past two years I have read or reread every one of Agatha Christie's 66 full-length murder mysteries, from The Mysterious Affair of Styles - the very first of her whodunits to feature Hercule Poirot - to Sleeping Murder, the very last of them to feature Miss Marple. This demented overdose was not the result of some nerdily completist fantasy or, as non-admirers of Christie may suspect, an uncontrollable bout of masochistic self-flagellation. It was because I was writing what I myself liked to think of as the 67th, The Act of Roger Murgatroyd, a celebration-cum-critique-cum-parody of what remains perhaps her most ingenious and celebrated thriller, The Murder of Roger Ackroyd, first published 80 years ago.

The experience was largely enjoyable. Though I discovered that she had produced rather more feeble whodunnits than I had remembered, the overall standard of her plotting impressed me as amazingly high for such a prolific writer. And her evocation of a utopian if also profoundly reactionary England - snobbery, racism, anti-semitism and all - continues to be so irresistible even to a modern reader that I have no problem understanding her enduringly high sales and the countless radio, television, stage and film adaptations.

Every Christie fan will be familiar with that sense of mounting tension as one approaches the climax of one of her books - the struggle, in particular, to keep one's eyes from straying too far ahead in case they catch, before they're meant to, the presiding sleuth's "And the name of the murderer is ..." I discovered that this tension is absolutely not inherent in the textures and trappings of the novels themselves. On the contrary, the latter chapters of most Christie whodunits are, if anything, even more platitudinously wordy than the earlier ones. There's no abrupt quickening of pace, no intensification of atmosphere, none of the habitual devices by which thriller writers progressively turn the screw.

It was at that moment, in fact, that I realised that the real tension resides exclusively in the reader's own mind. If an Agatha Christie novel appears to become increasingly suspenseful as it approaches its denouement, it's because the reader himself, already keyed-up, begins to grow as nervous as one of the suspects in the novel. After all, he has invested a fair amount of time and energy in the book and he can't bear the prospect of its climax proving to be a letdown, either because it's not clever enough or because it's too clever by half.

This curious transference of narrative tension from the text itself to the reader made me realise, too, that Christie is arguably a more modern writer - even a postmodern writer, as we used to say - than she's ever given credit for.

Consider a few of the more abstruse critical methodologies of the last four decades - psychoanalytical, semiological, ideological, etc. If there's a single characteristic shared by all of them, it's what might be called an allure of improbability.

On occasion, a reader will uncon-sciously express his appreciation of some book in terms more appropriate to this kind of theoretical analysis - because he has been prompted to do so by its author. Agatha Christie, unexpectedly, is just such an author.

As her publisher never tires of reminding us, Christie is a novelist whose books are rivalled in sales only by the Bible and the complete works of Shakespeare. Unlike them, though, her whodunits belong to a still slightly disreputable sub-species of literature. The last page turned, they tend to be casually left behind in railway carriages or abandoned on poolside loungers. They are tricks, one-night-stands. Yet, paradoxically, they elicit from even the most unthinking of consumers a genuinely critical response - critical in the specifically modern sense that the reader is alert throughout to the fact that what he is dealing with is a literary artefact, with something that first had to be written, to be produced.

A banal illustration, variants on which are to be found in virtually every Christie whodunit: a murder has been committed - naturally, the suspects are already assembled, and Hercule Poirot, usefully chancing to be in the vicinity, proceeds to investigate. It eventually transpires that a shadowy figure was glimpsed near the scene of the crime, a woman most likely, but a woman (the witness adds in a pensive afterthought) of strangely masculine build. Forty pages or so further on, we learn - softly, very softly, Christie's most cunning clues tending to tiptoe onto the page - that one of the male suspects had made, when an undergraduate, a memorable impression as a female impersonator in some end-of-term revue ...

The innocent reader, one not yet well-acquainted with Christie's narrative strategies, will as a rule respond to this seemingly corroborative piece of information by gleefully assuming that he has discovered the murderer's identity - a perfectly sensible form of deductive reasoning based, however, on a guileless reading of the novel. Which is to say, the whole chain of circumstances may be ridiculously improbable but, all in all, supposing these circumstances exist outside of a book, it's unquestionably his solution that would most logically account for them. If, in "real life", a mannishly built woman was witnessed leaving the scene of a crime and it was subsequently revealed that one of the likely suspects had formerly known success as a drag artist, then it's surely safe to say that no one would be surprised if that suspect was eventually revealed to be the criminal.

The experienced Christie reader knows better, of course. Duly registering the suggestiveness of the evidence, he either withholds judgment altogether or judges the clue too flagrant as a credible tip to that particular suspect's guilt. Or even, falling victim to a species of paranoia that whodunits deliberately induce, continues to entertain it precisely because he suspects that its "too flagrant" visibility is ultimately a ruse to persuade him to discard it. To the pendulum swing of such an unresolvable dialectics (aha, he did it! - no, no, he didn't, because Agatha Christie wants me to think he did - ah, but what if she actually wants me to think he couldn't have done it because it's too obvious a clue - which means that he could well have done it after all?) the only conceivable end is in the asylum.

In any event, and however frivolous the literary stake, such a reader can certainly claim to be subjecting the novel to an anti-illusionistic, therefore critical, reading. Not for an instant does he identify with Christie's characters, with their psychologies or motivations. In a way, he identifies with just one "character" - Agatha Christie herself.

Like certain postmodern novels to which they otherwise bear no resemblance, Christie's thrillers are honeycombed with authorial insinuations designed to warn her readers that the book in their hands is a product of human artifice. Identical twins invariably give one pause, for instance, as does the proximity, at the scene of the crime, of a clock and a mirror (with the implication that the dial may have been read in reverse), or any allusion to the black sheep of a respectable family shipped off in disgrace to Australia or South Africa and supposed (but not quite known) to have died there.

These are, to be sure, all whiskery conventions peculiar to the classic English whodunit, a genre of which Christie was to become by far the most successful practitioner. When the reader turns the last pages of a Christie whodunit (and here I ought to warn you that I'm about to reveal a trio of her most celebrated twists), and learns that the murderer of Roger Ackroyd is the novel's first-person narrator, or that the murder on the Orient Express was committed by not one but all of the suspects, or that the perpetrator of the apparently agent-less killings in And Then There Were None is one of the apparent "victims", he treats each revelation less as an intriguing insight into a pathological mindset than as a feat of pure literary ingenuity, to be judged according to the stimulation it affords solely from that point of view.

Implausibilities, psychological or other, cease to matter. What does matter is that, like two players hunched over a chessboard, reader and author lock themselves in combat, each openly acknowledging the adversary's existence and skill. And, at their best, Christie's denouements are comparable to elegant chess endgames, if of a type whose aphoristic concision has next to nothing to do with the authentic parameters of the game. (Chesterton, by contrast, limited the narrative scope of his Father Brown short stories to endgames alone, thereby sparing himself the ungrateful task of rendering their prehistory credible.)

The Christie reader is also, naturally, an armchair detective, a detective by proxy. He doesn't identify with Poirot but operates independently of him, sifting the various clues that have been strewn across his path by an author whom we can't help regarding as a murderess by proxy, a designation encouraged by her faintly ghoulish public image, of a bespectacled old dear with an incongruous partiality to homicide.

· The Act of Roger Murgatroyd is published this month by Faber, price £12.99. To order a copy for £11.99 with free UK p&p call Guardian book service on 0870 836 875.